Modify to spit out Firebase-friendly JWT?

I’m working on a mobile app in Flutterflow to serve as a front-end for my site. Flutterflow supports a JWT authentication method, which creates a user in a Firebase database. Basically, you use your site’s API to authenticate the user via username/password and to return a Firebase-friendly JWT, then the app stores that JWT in the Firebase database for your app.

Is there any way to use a filter with a custom REST endpoint to support creating a JWT like below?

Per the Firebase documentation, the code/payload to spit out a compliant JWT is

// Requires: composer require firebase/php-jwt
use Firebase\JWT\JWT;

// Get your service account's email address and private key from the JSON key file
$service_account_email = "[email protected]";
$private_key = "-----BEGIN PRIVATE KEY-----...";

function create_custom_token($uid, $is_premium_account) {
  global $service_account_email, $private_key;

  $now_seconds = time();
  $payload = array(
    "iss" => $service_account_email,
    "sub" => $service_account_email,
    "aud" => "https://identitytoolkit.googleapis.com/google.identity.identitytoolkit.v1.IdentityToolkit",
    "iat" => $now_seconds,
    "exp" => $now_seconds+(60*60),  // Maximum expiration time is one hour
    "uid" => $uid,
    "claims" => array(
      "premium_account" => $is_premium_account
    )
  );
  return JWT::encode($payload, $private_key, "RS256");
}